it has to be made from cold drawn or seamless tube and specific diamters and thicknesses for different parts of the structure.

Best option is to use a CAMS approved design (in your Motorsport Manual), otherwise the cage needs to be 'homologated' with engineer testing to comply with their standards.

if you are asking these kinds of questions, then I think you should take it to an experienced builder.

But I think the first thing you should do is join your State's Improved Production Car Club and talk to the other guys. Then get hold of a CAMS motorsport manual and study the rules before you lift a spanner.